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Recommendations for TCE Plume Action
Overview


This section provides an overview of the available technical information, and rationale for the plume action recommendations presented in this report.

Rationale

Recommendations on the TCE problem in the south valley area were requested by the Mayor to provide guidance for designing a performance-based RFP for remediation alternatives, and to help the city council make critical decisions based on the proposals that are expected to be submitted under the RFP. The philosophy behind these recommendations is based on stressing factual information only, making maximum use of available information to prioritize needs, and identifying critical missing information that will be needed to design a long-term solution.

 

Problem

Dissolved TCE solvent has been found in 8 wells since 1993, when detailed monitoring commenced. In 1993, only three wells had solvent concentrations above 2 parts per billion (ppb); by 1998, six wells had concentrations above 2 ppb, and average concentrations in the period 1996-98 had grown 13% compared to 1993-95. Detailed mapping of solvent distribution conducted in 1994-95 and the geographic pattern of other affected wells, indicates that a solvent plume occupies a large portion of the aquifer along the western half of the southern valley and its total solvent mass has grown steadily between 1993 and 1998 (Figures 1a and 1b, respectively). There seems to be little doubt that solvent contamination in individual wells and the contamination level in the aquifer are growing. Although contaminant levels in wells with the highest concentrations have varied since 1993 (Figure 2a), there is no evidence that concentrations are decreasing (Figure 2b), and the geographic pattern of wells affected by increasing concentrations indicates that the plume is growing (Table 1; compare Figures 1a and 1b). Perhaps most tellingly, at least 100 gallons of liquid solvent have already moved past the LDS ballfield since 1993, indicating that the source(s) responsible for this problem have deposited a large quantity of solvent into the subsurface with the potential for continued high concentrations and little likelihood for the plume's diminishment in the near future.

 

Recommendations

Since the southern valley provides most of the ground water which supplies the entire municipal well field, and the portion of the aquifer from which clean water can be produced is shrinking as the solvent plume grows, the Forum Working Group makes two principal recommendations:

  1. reduce solvent content in ground water to protect downstream wells from contamination (a process known as "Plume Control"); this could be accomplished by a variety of methods including, but not limited to, pump-and-treat (aeration towers), in situ aeration and vacuum extraction (sparging), and bioremediation; and

  2. identify the extent of the "Data Gap" in the south valley (the area on the west side between the mouth of Mink Creek and Hillside Lane in which the plume has not been traced), so as to identify and characterize the source(s) from which solvent is emanating, to define future trends, and to evaluate the performance of future remediation efforts.

Recommendation 1 addresses the urgency of the water supply and water quality situation in the south valley, where options for developing alternative clean supplies are already limited because of the narrow geographic extent of the productive aquifer, the extent and proliferation of existing contamination, and potential impacts of areas that are vulnerable to surface land uses (e.g. Highway Pond). The Working Group recommends plume control as the highest priority to ensure future management options for delivering safe drinking water to customers. Without plume control, the city will have fewer options and less flexibility in where to drill new wells to supply clean drinking water to meet future demand. For example, the LDS ballfield pump-and-inject wells would counteract some of the tendency of Well 44 to draw contamination across the aquifer (Appendix 3), thereby helping to reduce the spread of solvent contamination due to production from new supply wells.

Recommendation 2 reflects the need to develop a long-term remediation strategy based on "source control", which must be predicated on better knowledge of the contaminant source(s) responsible for the plume. Without more than circumstantial evidence to go on, it would be imprudent to design a remediation scheme on the assumption that the landfill is the sole source. On the other hand, our best available information for remediation design anywhere in the aquifer is currently in the LDS ballfield area. Although it is not a solvent source area per se, it is the only known location that we can presently address with confidence and also the one location in the aquifer where engineering confidence is highest that plume control will be effective.